In the United States steel industry, direct current (DC) motors having drum-shoe brakes are predominantly used for crane hoist drives. Such drum-shoe brakes pose a significant maintenance problem for the steel mills due to the necessity of frequent adjustments, the existence of thermal limitations, and the extreme difficulties encountered during servicing. Until presently, caliper disk brakes have not been employed in these applications for a number of reasons, among which is the American Iron and Steel Engineers (AISE) specification that the brake coil must be wired in series and must accommodate all of the electric current to the motor of the crane hoist drive. As a consequence, extremely large coils and armatures for releasing the brake are necessitated--significantly increasing the difficulty of adapting a caliper type of brake design. Additionally, the DC motor shaft has a significant amount of axial movement, on the order of 3/16 inch, which the brake must accommodate. Such axial movement poses a significant problem for caliper disk brakes of the necessary size, particularly with respect to drag and wear.
Additionally, drum-shoe brakes are given to high wear rates, requiring frequent servicing and shoe replacement. Typically, these brakes are in areas which are difficult to access, being associated with cranes in elevated locations. The brake shoes are both heavy and cumbersome. Indeed, in the prior art the process of shoe replacement was not only frequently undertaken, but was a lengthy one.
The position and area allotted for crane drive brakes is rather fixed, requiring that the brake fit into a defined and somewhat limited envelope. Accordingly, any improvement or modification to the brake system must necessarily be confined to that same envelope.
As shown in FIG. 1, a prior art electromagnetic drum brake is designated generally by the numeral 10. The drum brake 10 includes a base 12 maintaining at one end thereof, a pair of armatures 16, 18 having an electromagnetic coil 14 interposed therebetween, the armatures 16, 18 and the electromagnetic coil 14 defining a magnet assembly. A torque spring 20 is also interposed between the inner armature 16 and the outer armature 18, the spring 20 urging the two armatures apart. As is well known and understood by those skilled in the art, the armatures 16, 18 are pivotally or flexibly maintained at the bottoms thereof at the base 12 by angle 15 plates to operate in a clam shell fashion. However, such angle plates were given to fatigue and routinely failed.
An inner shoe lever 22 and outer shoe lever 24 respectively receive inner and outer shoe linings 26, 28. The inner shoe lever 22 is connected to the inner armature 16 by means of an appropriate connector 30, while a pull rod 32 interconnects the outer shoe lever 24 and the outer armature 18, as shown. The shoe linings 26, 28 are maintained in diametric opposition with respect to the drum 34. The drum 34 is rotatable by means of a hub 36 having an aperture and key way 38 adapted for securely receiving the shaft of a crane hoist drive.
In operation of the prior art, with the electromagnetic coil 14 energized, the armatures 16, 18 are drawn together, pulling the brake shoes 26, 28 away from the rotating drum 34, allowing the crane hoist drive to operate freely. However, when the coil 14 is de-energized, the torque spring 20 forces the armatures 16, 18 apart, urging the brake shoes 26, 28 into frictional contacting engagement with the drum 34 to inhibit rotation of the drum 34 and the attached shaft.
In the prior art brake 10, the shoes 26, 28 are heavy, cumbersome, and given to rapid wear. Additionally, the structure of the brake assembly 10 is such that removal and replacement of the brake shoes 26, 28 require substantial dismantling and disassembly of the brake assembly 10. Accordingly, it is desirable to devise a disk brake assembly in substantially the same general envelope as that employed by the brake 10, and employing the same armature and coil assembly and various other structural features of the prior art.
The drum-shoe brakes of the prior art are generally characterized by a limited thermal capacity, restricting the operability of the brakes, since every brake operates on the basis of converting mechanical energy to thermal energy. Accordingly, it is desirable to devise a disk brake assembly having increased disk diameter over the drum-shoe brakes of the prior art, thus accommodating higher duty cycles of operation than with the prior art. The increased productivity demands of the steel industry can only be met by the provision of a brake that accommodates such high duty cycles and which can be placed in the same space as the prior art brakes.